Method for demodulating a digital signal based on an estimate of multipath propagation impairment, and an associated receiver

ABSTRACT

A method for demodulating a received digitally modulated signal subjected to multipath propagation impairment includes estimating the multipath propagation impairment of the received digitally modulated signal using a channel estimator, and estimating at least one symbol of the received digitally modulated signal using a symbol estimator. The at least one estimated symbol is adjusted based upon the estimated multipath propagation impairment to generate an estimate of the at least one symbol as impaired by the multipath propagation. At least one error signal is generated by comparing the estimate of the at least one symbol as impaired by the multipath propagation to the received digitally modulated signal. The at least one error signal is used for estimating remaining symbols to be demodulated and for refining the estimated multipath propagation impairment.

RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.09/866,539 filed May 25, 2001 now U.S. Pat. No. 7,006,581 which is basedupon prior filed copending provisional application No. 60/207,028 filedMay 25, 2000, the entire contents of which are incorporated herein byreference.

FIELD OF THE INVENTION

The present invention relates to the field of digital communications,and more particularly, to demodulation of a serially modulated signalsubjected to multipath propagation impairment.

BACKGROUND OF THE INVENTION

A phenomenon in wireless communication systems, such as digital radio ortelevision transmission, is multipath propagation. This type of signaldegradation occurs when a broadcast signal takes more than one path fromthe transmitting antenna to the receiving antenna so that the receivingantenna receives multiple signals. One of these multiple signals maycome directly from the transmitting antenna, but several other signalsare first reflected from buildings and other obstructions beforereaching the receiving antenna, and are thus delayed slightly in phasefrom one another.

The reception of several versions of the same signal shifted in phaseresults in a composite signal actually being received at the receivingantenna. Two techniques may be used to deal with the multipathpropagation of digitally modulated signals. These two techniques areinverse equalization and maximum likelihood sequence estimation (MLSE)detection.

In inverse equalization, an equalizer is implemented, digitally orotherwise, to reverse the propagation effects of multipath on thetransmission waveform prior to detection. The equalizer is trained usingblind equalization methods, decision feedback methods or by atransmitted training waveform.

There are two fundamental limitations of inverse equalization. The firstis the equalizer length, which is a function of the multipathpropagation impairment characteristics, namely echo delay and echoamplitude. Equalizer length is necessarily equal to or greater than, andoften many times greater than, the multipath delay spread, depending onthe amplitude of the multipath pre-echo and/or post-echo components. Thesecond fundamental limitation of inverse equalization is that of 0 dBecho performance. In cases where the amplitudes of delayed signals areequal or nearly equal, the necessary equalizer is usually eitherunrealizable or impractical.

In MLSE detection systems, a fundamental limitation is complexity. Incases where the channel path count is large and the delay spread is muchgreater than the symbol interval, the list of survivors becomesunmanageably large, as does the length of the trellis required torepresent each survivor. For example, several MLSE detection systemshave been disclosed, such as the ones in Parr et al. (U.S. Pat. No.5,263,026), Polydoros et al. (U.S. Pat. No. 5,432,821) and Parr et al.(U.S. Pat. No. 5,471,501).

In the Parr et al. '026 patent, a method for MLSE demodulation of areceived serially modulated signal is disclosed, wherein multipathpropagation impairment characteristics are estimated using a least meansquare (LMS) algorithm. Rather than converging on an inverse of themultipath propagation impairment, the LMS algorithm converges on anestimate of the multipath propagation impairment. This channel estimateis integrally incorporated into the MLSE algorithm used to determine thesymbols making up the serially modulated signal.

In the Polydoros et al. '821 patent, multipath propagationcharacteristics are incorporated into the survivor selection processused to accomplish data sequence selection. The survivor selectionprocess is likewise based upon MLSE detection. Also in the Parr et al.'501 patent, MLSE detection is performed using an estimation of themultipath propagation impairment. As discussed above, the MLSEdemodulation approach is limited by complexity.

A high definition digital television (HDTV) signal is also susceptibleto multipath propagation impairment. The HDTV signal is a seriallymodulated signal based upon the standard set by the Advanced TelevisionSystem Committee (ATSC) for terrestrial broadcast television in theUnited States. The ATSC digital television standard was determined bythe Grand Alliance and was subsequently accepted by the broadcastcommunity, the consumer electronics industry and the regulatoryinfrastructure.

The regulatory infrastructure has mandated a strictly scheduledtransition of terrestrial broadcast television in the United States fromthe National Television System Committee (NTSC) or “analog” standard tothe ATSC or “digital” standard. A significant investment is in place onbehalf of the broadcast industry to support this planned transition.Similarly, many consumers have purchased ATSC television receiverequipment that include new ATSC system complaint DTV television sets andDTV television set-top converters.

However, the ATSC standard, in its present form, is deficient in itssusceptibility to multipath propagation impairment. In side-by-sidecomparisons, ATSC reception, i.e., the new digital system, is ofteninferior to NTSC reception, i.e., the conventional analog system.Additionally, ATSC mobile reception suffers substantially moredegradation due to multipath propagation impairment than NTSC mobilereception. Signal strength and signal-to-noise (SNR) ratios aretypically not an issue, as unanticipated inferior reception manifestsitself at high levels of received signal power and at high receiver SNRratios. This fact, coupled with spectral analysis of received ATSC DTVsignals, points directly to multipath propagation impairment as thecause of the inferior reception.

Various efforts have been made in the area of DTV reception. Forexample, Park et al. (U.S. Pat. No. 5,592,235) discloses combiningreception, appropriate to terrestrial broadcast and to cable broadcast,both in a single receiver. Also included in these various efforts isOshima (U.S. Pat. No. 5,802,241), which discloses a plurality ofmodulation components modulated by a plurality of signal components.Both of these references disclose the use of equalization. As discussedabove, complexity of an equalizer is a fundamental limitation.

With respect to enabling the initial acquisition of digitally modulatedsignals that are severely distorted by multipath propagation impairment,decision-feedback equalizers (DFE) are not suitable. For this purpose, areference or training waveform is typically introduced. The use of areference sequence equalizer for equalizing GA-HDTV signals is disclosedin Lee (U.S. Pat. No. 5,886,748). Unfortunately, the Lee '748 patentdoes not overcome the limitations associated with inverse channelequalizers.

SUMMARY OF THE INVENTION

In view of the foregoing background, it is therefore an object of thepresent invention to provide a method for demodulating a receiveddigitally modulated signal that is subjected to multipath propagationimpairment, particularly when multiple signals of the received signaldefining the multipath propagation impairment are substantially equal toone another.

Another object of the present invention is to provide a correspondingdigital receiver that is relatively straightforward to implement fordemodulating the received digitally modulated signal.

These and other objects, advantages and features in accordance with thepresent invention are provided by a method for demodulating a receiveddigitally modulated signal subjected to multipath propagationimpairment. The method preferably comprises estimating the multipathpropagation impairment of the received digitally modulated signal usinga channel estimator, and estimating at least one symbol of the receiveddigitally modulated signal using a symbol estimator.

The method preferably further includes adjusting the at least oneestimated symbol based upon the estimated multipath propagationimpairment to generate an estimate of the at least one symbol asimpaired by the multipath propagation, and at least one error signal isgenerated by comparing the estimate of the at least one symbol asimpaired by the multipath propagation to the received digitallymodulated signal. The at least one error signal is then preferably usedfor estimating remaining symbols to be demodulated.

The method preferably further comprises using the at least one errorsignal for refining the estimated multipath propagation impairment.Next, the method also preferably further comprises estimating at leastone next symbol, and adjusting the estimate of the at least one nextsymbol based upon the refined estimated multipath propagation impairmentfor generating an estimate of the at least one next symbol as impairedby the multipath propagation.

The at least one error signal is preferably refined by comparing theestimate of the at least one next symbol as impaired by the multipathpropagation to the received digitally modulated signal. Refining the atleast one error signal preferably further comprises comparing theestimate of the at least one next symbol as impaired by the multipathpropagation to the at least one error signal resulting from at least oneprevious comparison.

Estimating the multipath propagation impairment may be based upon anadaptive algorithm, or based upon a training waveform embedded in thereceived digitally modulated signal. Similarly, estimating the at leastone symbol may be based upon an adaptive algorithm, or based upon thetraining waveform embedded in the received digitally modulated signal.With respect to the adaptive algorithms, each algorithm may comprise arespective least mean square (LMS) algorithm that has applied thereto aconvergence coefficient. The convergence coefficient is preferably basedupon the received digitally modulated signal.

After the at least one symbol has been estimated, the remaining symbolsto be demodulated are preferably estimated based upon linear estimation.This is performed based upon the at least one error signal. In otherwords, linear estimation of the remaining symbols or adaptive estimationof the remaining symbols allows the received digitally modulated signalto be demodulated when impaired by multichannel propagation,particularly when multiple signals of the received signal defining themultipath propagation impairment are substantially equal to one another.

Since possible combinations of the symbols to be demodulated arepreferably not estimated, as is typically the case for a MLSE equalizer,the complexity of a digital receiver demodulating the received digitalsignal is minimized. Consequently, performing an adaptive estimation ora linear estimation for the symbols to be demodulated overcomes thelimitations applicable to inverse equalization and MLSE estimation, asdiscussed in the background section.

The received digitally modulated signal preferably comprises at leastone of a digital broadcast television signal, a digital broadcast radiosignal, a digital cellular telephone signal, and a digital wirelesslocal area network (LAN) signal. Of course, the method according to thepresent invention may also be applied to other radio systems and tocommunication through various types of media. In addition, the receiveddigitally modulated signal may be a digitally serial modulated signal.

Another aspect of the invention is directed to a method forsimultaneously demodulating a plurality of received digitally modulatedsignals subjected to multipath propagation impairments. The methodpreferably comprises estimating the multipath propagation impairments ofthe plurality of received digitally modulated signals using a pluralityof channel estimators, and estimating at least one symbol of each of theplurality of received digitally modulated signals using a plurality ofsymbol estimators.

Each estimated symbol is preferably adjusted based upon thecorresponding estimated multipath propagation impairment to generate anestimate of each symbol as impaired by the corresponding multipathpropagation, and at least one error signal is preferably generated bycomparing a summation of the estimates of the symbols as impaired by thecorresponding multipath propagation to the plurality of receiveddigitally modulated signals. The at least one error signal is preferablyused for estimating remaining symbols of each of the plurality ofreceived digitally modulated signals to be demodulated.

Another aspect of the present invention is directed to a receiver fordemodulating a received digitally modulated signal subjected tomultipath propagation impairment. The digital receiver preferablycomprises a channel estimator for estimating the multipath propagationimpairment of the received digitally modulated signal, and a symbolestimator connected to the channel estimator for estimating at least onesymbol of the received digitally modulated signal.

The channel estimator preferably adjusts the at least one estimatedsymbol based upon the estimated multipath propagation impairment togenerate an estimate of the at least one symbol as impaired by themultipath propagation. The digital receiver may further comprise asumming network connected to the channel estimator and to the symbolestimator for generating at least one error signal by comparing theestimate of the at least one symbol as impaired by the multipathpropagation to the received digitally modulated signal. The symbolestimator preferably uses the at least one error signal for estimatingthe remaining symbols to be demodulated.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified block diagram of a digital transmitter includinga continuous-time modulator and a channel model in accordance with theprior art.

FIG. 2 is an illustration of a segment of a digitally modulated waveformcomprising a plurality of symbols in accordance with the prior art.

FIG. 3 is an illustration of various physical objects providingpropagation paths for a transmitted signal in accordance with the priorart.

FIG. 4 is an illustration of a five-signal multipath model being appliedto a digitally modulated signal in accordance with the prior art.

FIG. 5 is a simplified block diagram of a digital transmitter includinga time-sampled modulator and a channel model in accordance with theprior art.

FIG. 6 is a block diagram on the architecture of a digital receiverbased upon equalization in accordance with the prior art.

FIG. 7 is an illustration of a two-signal multipath model having abenign multipath being applied to a digitally modulated signal inaccordance with the prior art.

FIG. 8 is an illustration of the successful equalization of a receivedsignal impaired by a moderate two-signal multipath model in accordancewith the prior art.

FIG. 9 is an illustration of the failure of conventional equalizationwhen a received signal impaired by a severe two-signal multipath modelis applied thereto in accordance with the prior art.

FIG. 10 is an illustration on the 0 dB echo problem, both static anddynamic, to conventional equalizers in accordance with the prior art.

FIG. 11 is a flow diagram for demodulating a received digitallymodulated signal in accordance with the present invention.

FIG. 12 is a simplified block diagram of a digital receiver illustratingthe cooperation between symbol estimation and channel estimation inaccordance with the present invention.

FIGS. 13-16 are illustrations of demodulation of the first six symbolsof a received signal impaired by multipath propagation, with thedemodulation based upon linear estimation in accordance with the presentinvention.

FIG. 17 is a block diagram of a digital receiver having adaptive channelestimation in accordance with the present invention.

FIG. 18 is a block diagram of a digital receiver having adaptive symbolestimation in accordance with the present invention.

FIG. 19 is a block diagram illustrating a digital receiver having jointadaptive channel estimation and symbol estimation in accordance with thepresent invention.

FIG. 20 is a detailed block diagram of a digital receiver having jointadaptive channel estimation and symbol estimation associated with aplurality of independent modulation sources in accordance with thepresent invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will now be described more fully hereinafter withreference to the accompanying drawings, in which preferred embodimentsof the invention are shown. This invention may, however, be embodied inmany different forms and should not be construed as limited to theembodiments set forth herein. Rather, these embodiments are provided sothat this disclosure will be thorough and complete, and will fullyconvey the scope of the invention to those skilled in the art. Likenumbers refer to like elements throughout and prime and multiple primenotations are used in alternate embodiments. The dimensions of layersand regions may be exaggerated in the figures for greater clarity.

Referring initially to FIGS. 1-10, a digital transmitter and a digitalreceiver of the prior art will be discussed, including the impact ofmultipath propagation on a digitally modulated signal. A simplifiedblock diagram of a digital transmitter 10 including a continuous-timemodulator 12 and a channel modeler 14 is illustrated in FIG. 1.

In the digital transmitter and channel model 10, x(n) represents thedata sequence applied to a modulator 12, which generates a modulatedwaveform s(t) represented in real time. The modulated waveform isbroadcast through a propagation channel 14 having a time response h(t,τ)in a convolutional continuous-time domain τ that varies continuouslyover time t. Noise n(t) is added via a summing network 16 for generatingthe resulting waveform, which is represented by the waveform r(t).

Referring now to FIG. 2, a modulated waveform 18, for example, comprisesa series of different modulation symbols or symbols 18 ₁-18 ₆. Themodulation symbols 18 ₁-18 ₆ may also be referred to simply as symbols.Each symbol is selected from an ensemble of unique shapes, i.e., ofvarying amplitudes and phases. Each unique shape represents a digitalstate or group of digital information bits.

These symbols 18 ₁-18 ₆ are transmitted serially, i.e., one right afterthe other. Digital serial modulation is contrasted with OrthogonalFrequency Division Multiplexing (OFDM/COFDM) in that serial modulationcarries information serially while OFDM/COFDM carries information bothserially and across the modulation spectrum. Although OFDM/COFDM canoffer multipath propagation advantages, digital serial modulation issuperior in that it is simpler and does not suffer from distortion dueto extreme ratios of peak-to-average power.

In an ideal world, digital transmission passes through a medium, such asair or space, in a straight line and unimpaired. As illustrated in FIG.3, a transmitter 30 transmits a signal via transmit antenna 32 toreceive antenna 34, which is connected to a receiver 36. Realistically,however, the transmitted signal is subjected to obstacles. Thetransmitted signal is reflected from objects such as buildings 20,bridges 22, aircraft 24, and other man-made and natural structures orobstacles 26. Consequently, the transmitted signal arrives at thereceiver 36 after having passed through any number of multiple paths,such as any one of the five paths illustrated in FIG. 3.

For a clear path, a clear channel response may be represented as asingle signal component 40 ₁, as illustrated in FIG. 4. The singlesignal component 40 ₁ indicates a single time of arrival (TOA), withtime progressing from left to right. A multipath response is indicatedby multiple signal components 40 ₁-40 ₅, with each signal componentindicating a different arrival time, a different amplitude and adifferent phase which may be either positive or negative. In theillustrated example, there are five paths in the transmission medium atsome instant in time. Each signal component corresponds to one of thesepaths.

Assuming that a single symbol 18 ₁ from the received signal travelsacross a single path, then it is received at a single arrival time aspart of signal component 40 ₁. This arrival time corresponds to a singledelay and at the amplitude and phase associated with the first singlepath. However, in a multipath situation, the second path contributes acomponent 18 _(1A) to the received signal (i.e., there is another symbol18 ₁ provided by multipath signal component 40 ₂) at a second delay witha second associated amplitude and phase. Likewise the third pathcontributes a component 18 _(1B) to the received signal (i.e., there isanother symbol 18 ₁ provided by multipath signal component 40 ₃), thistime with a negative phase. Similarly, the fourth and fifth paths eachcontribute a component 18 _(1C), 18 _(1D) to the received signal (i.e.,there are two more symbols 18 ₁ provided by multipath signal components40 ₄ and 40 ₅).

What the receiver 36 sees is the sum of these five multipath components,as represented by signal 50, which is distorted compared to the originaltransmitted symbol 18 ₁. The assumption is now made that an entiredigital serial modulation waveform is transmitted 18 to include sixconsecutive symbols 18 ₁-18 ₆. In this case, the received signal 19 isdistorted by the presence of five separate paths in such a way as tocause the signal 18 to interfere with itself. The received signal 19 isunrecognizable in this case due to the impairment by the multipathpropagation.

When the receiver and demodulation techniques are implemented digitally,digital equalization and multipath analysis lend themselves tosampled-time digital modeling and analysis. As such, the digitaltransmitter and channel estimate 10′ illustrated in FIG. 5 includes amodulator 12′ and a channel modeler 14′, which are represented insampled-time as compared to continuous-time shown in FIG. 1. In thisillustration, continuous-time t is replaced by time-sampling index n andthe continuous convolutional-time domain τ is replaced by thetime-sampling convolutional index m.

This model allows for complex (real and imaginary) signal representationand for time sampling intervals which may be integer fractions of thesymbol interval. In this model, the same transmission data sequence x(n)as that in FIG. 1 is applied to a time-sampling digital modulator 12′yielding the time-sampled modulated waveform s(n). The time-sampleddigitally modulated waveform s(n) is applied to the time-sampled channelmodel h(n,m) 14′, which is made up of a sequence of time-sampled impulseresponses in index m, one per time index n.

Time-sampled noise n(n) is added via a summing network 16 to the outputof the time-sampled channel or multipath model process h(n,m) to yield atime-sampled representation of the received modulated waveform r(n),again in time index n. Successful demodulation requires sufficientconsideration of channel distortion h(n,m) in the process of estimatingthe modulation data sequence x(n).

Referring now to FIG. 6, an equalization process or method for a digitalreceiver 60′ will be discussed. An equalizer 62 is connected to ademodulator 64. An approximation {circumflex over (h⁻¹)}(n,m) to theinverse h⁻¹ (n,m) of the channel response h(n,m) is applied to thereceived waveform r(n). The resulting output ŝ(n) is an estimate of theoriginal modulation waveform s(n). The demodulator 64 operates on themodulation waveform estimate ŝ(n) to produce an estimate {circumflexover (x)}(n) of the modulation data sequence x(n).

Provided that the channel-inverse equalization response h⁻¹ (n,m) existsand can be approximated sufficiently as {circumflex over (h⁻¹)}(n,m)within practical implementation limitations, such as finite impulseresponse (FIR) filter duration and resolution, the output {circumflexover (x)}(n) of the demodulator 64 will be a sufficiently accuratereproduction of the modulation data sequence x(n). However, equalizerlength, equalizer tap resolution and the existence and/or practicalimplementation of the inverse channel response are factors that effectthe practical implementation of the equalization process.

The operation and consequent limitations of conventional equalizertechniques will now be described with an example. A straightforwardexample of digital equalization based on a two-signal multipath channelis illustrated with reference to FIG. 7. Again, one starts with a clearpath which exhibits the clear channel response. The single signalcomponent 40 ₁ indicates the first single TOA, again with timeprogressing from left to right.

In the two-signal multipath response, each signal indicates a differentarrival time with a different amplitude and phase. Here we show twosignal components 40 ₁ and 40 ₂, with each signal componentcorresponding to one of two propagation paths in this example. Assuminga single symbol 18 ₁ travels across a single path, it is received at asingle arrival time corresponding to a single delay and at the amplitudeand phase associated with the first single path 40 ₁.

In a two-signal multipath situation, the second path contributes asecond component 18 _(1A) to the received signal (i.e., there is anothersymbol 18 ₁ provided by multipath signal component 40 ₂) at the seconddelay with a second associated amplitude and phase. What the receiver 36sees is the sum (signal 50′) of these two multipath components which isdistorted compared to the original transmitted symbol.

An assumption is now made that an entire digital serial modulationwaveform 18 is transmitted to include the six consecutive symbols 18₁-18 ₆. In this case, the received signal 19′ is distorted by thepresence of two distinct paths in such a way as to cause the signal tointerfere with itself. The received signal 19′ is severely distortedwhen compared to the original modulated signal 18.

Currently, receivers compensate for this multipath propagationimpairment, i.e., distortion, using equalization techniques. Consideringthe same transmitted serial modulated waveform 18, along with the sametwo-signal multipath response example as discussed above with referenceto FIG. 7, the received signal 19′ as shown earlier is again shown withreference to FIG. 8.

Equalization, as readily understood by one skilled in the art, employs afinite impulse filter (FIR) 62 for the received signal 19′, which isassumed to have a dominant primary path component 40 ₁. This filter (orequalizer) 62 operates on the principle of adding delayed versions ofthe received signal so as to cancel non-primary paths of lesserstrength.

In the example illustrated in FIG. 8, the equalizer begins byintroducing a delayed component 70 ₁ to the primary received signalcomponent 70 ₀. The delayed signal component 70 ₁ is designed to cancelthe secondary multipath component 40 ₂, which is smaller in amplitudewith respect to the primary multipath component 40 ₁. The result is asignal 21 with most of the multipath distortion cancelled.

However, there is still some residual distortion at twice the echodelay. So the equalizer is adjusted by adding a tap 70 ₂, this time tocancel the compound echo at twice the path delay. The result is a muchcleaner signal, as illustrated by signal 21′. This may be repeated withtwo more taps 70 ₃₋₄ to produce an even cleaner signal 21″. Theresulting equalized waveform 21″ is very clean, almost indistinguishablefrom the modulated waveform 18.

Unfortunately, equalization may not be sufficient when the echo isalmost as strong as the direct path signal. Referring now to FIGS. 9 and10, the two-signal 40 ₁ and 40 ₂ multipath scenario will be addressedagain, except this time multipath signal component 40 ₂ is almost asstrong as the direct signal component 40 ₁. Each signal component 40 ₁and 40 ₂ corresponds to one of two propagation paths.

With a six-tap 70 ₀₋₅ equalizer, the resulting signal 25 has themultipath propagation impairment cancelled at the echo and out to fourcompound echoes. However, there is a great deal of residual noise, notevident on the left, where cancellation is illustrated, but on theright, where the compound echoes go uncancelled. This example is carriedout to a nine-tap 70 ₀₋₈ equalizer which passes the received signal 23(first tap 70 ₀), cancels the channel echo (second tap 70 ₁) and cancelsseven subsequent compound echoes 70 ₂₋₈, out to 8 times the originalpath delay, as indicated by signal 25′.

The result again shows cancellation on the left, but there is stillsignificant noise remaining, as indicated on the right. However, a morerealistic picture of what is happening is made available when one addsthe effect of the multipath and the equalizer on the symbols arrivingbefore the six 18 ₁-18 ₆ that are illustrated in the digital serialmodulated waveform 18. The resulting signal 27 is as bad as, if notworse, than the original received waveform 23.

The equalization process has another problem with respect to the 0-dBecho, as illustrated with reference to FIG. 10. Considering themultipath profile where two signal components 40 ₁ and 40 ₂ are veryclose in amplitude, with the first signal component 40 ₁ dominating. Thenecessary equalizer response would be a “post” equalizer, which cancelsthe second component 40 ₂ with respect to the first component 40 ₁.

Suppose now that the multipath response were to change, and the secondsignal component 40 _(2B) began instead to dominate the first signalcomponent 40 _(1B). This is because the first signal component sufferedattenuation, or because the first signal was blocked and both pathsrepresent reflections. In this case, the multipath cancellation requiresa “pre” equalizer filter, cancelling the first signal component 40 ₁ toarrive with respect to the second signal component 40 ₂.

As discussed in the background section, these equalizers are long, muchlonger than their corresponding path delays. This characteristic makesthem difficult to implement. As a practical matter, each additionalrequired equalizer tap introduces additional noise into the system. Themore taps, the more difficult it is to demodulate, even when theequalizer can implement all the taps. The discontinuity from the “post”equalizer to the “pre” equalizer represents a very difficult equalizertraining problem. When the multipath response has two equal signalcomponents 40 _(1A) and 40 _(2A), equalization can not be used.

The present invention will now be described with reference to FIGS.11-20. Referring to the flow chart illustrated in FIG. 11, from thestart (Block 90) the method for demodulating a received digitallymodulated signal that is subjected to multipath propagation impairmentcomprises estimating the multipath propagation impairment of thereceived digitally modulated-signal using a channel estimator at Block92, and estimating at least one symbol of the received digitallymodulated signal using a symbol estimator Block 94.

The method further includes adjusting the at least one estimated symbolbased upon the estimated multipath propagation impairment to generate anestimate of the at least one symbol as impaired by the multipathpropagation Block 96, and at least one error signal is generated bycomparing the estimate of the at least one symbol as impaired by themultipath propagation to the received digitally modulated signal atBlock 98. In other words, the initial symbol sequence estimate isconvolved with the multipath estimate, and the result of the convolutionis subtracted from the received signal to generate the at least oneerror signal. The at least one error signal is then preferably used forestimating remaining symbols to be demodulated at Block 100, and themethod may be stopped at Block 102.

The method according to this embodiment of the present inventionadvantageously combines channel estimation and symbol estimation fordemodulating the received digitally modulated signal, which may beserial. This avoids the limitations inherently associated with inverseequalization and MLSE detection as discussed above. The method may beused to successfully demodulate in the presence of all the multipathprofiles that can be corrected with an equalizer. In addition, thereceived signal may also be successfully demodulated in the presence ofall the multipath profiles that can not be corrected with an equalizerwithout requiring extremely long processing for multiple compounddelays, or without requiring special processing to accommodatediscontinuities as required by the equalizer. In other words, “killer”equalizer tracking problems are avoided with the method according to thepresent invention. There is also an increased signal-to-noise ratioadvantage in the present invention due to a reduction of required taps.

The present invention thus overcomes the dilemma of implementing apossibly non-existent inverse-channel response and reduces theresolution required of the associated processing with respect to thatrequired of comparable channel-inverse equalization techniques.

Referring now to the digital receiver 120 illustrated in FIG. 12, thetwo parts include symbol estimation using a symbol estimator 122 andmultipath estimation using a channel estimator 124. Initial multipathestimation may be as straightforward as correlating against a referencesequence like an a-priori PN sequence, as readily understood by oneskilled in the art, whereas symbol estimation can be as straight-forwardas linear combination or demodulation of the error vector, as alsoreadily understood by one skilled in the art.

Cooperative channel estimating demodulation will first be discussed. Theserial modulated waveform 18 used in previous examples will again be thecenter point of the discussion. In addition, the five-path multipathprofile 40 ₁₋₅ shown earlier will also be the center point of thediscussion.

The received signal 19 is stored in a memory 126. Suppose one coulddetermine or at least estimate what the multipath profile looked like130 ₁₋₅ by estimating the relative delay, amplitude and phase of everypath. Suppose also that one could search for or recognize the firstsymbol 18 ₁ in the received waveform 19.

Then, knowing the multipath profile 130 ₁₋₅ or at least having a goodappreciation as indicated by signal 132, one could assess the effects ofthis multipath profile on the first symbol 131, as illustrated in FIG.13. By subtracting this multipath-corrupted first symbol 18 ₁ from thereceived waveform 19 using a summing network 128, one gets an errorsignal 134.

In actuality, the first symbol 18 ₁ was recognized above by choosing thesymbol 131 which minimized this error waveform 134. We continue todemodulate this same serial modulated waveform 18. We already know thefirst symbol 18 ₁, and we are working off of the error signal 134derived from the previous step, and we have a good estimate of themultipath response 130 ₁₋₅.

In fact, we use the first symbol 18 ₁ to refine our good estimate of themultipath response and make it better. The next step is to estimate thesecond symbol 137 again by driving the estimation process, which causesconvergence of the error signal 134 to a set level, such as zero.

Application (e.g., convolution) to the multipath estimate yields anestimate 138 of the component of the received waveform which correspondsto the second symbol 18 ₂. Subtraction yields a new error signal 140,which is closer to flatline than the previous error signal 134, asillustrated in FIG. 14. This means we are making progress and that weare heading in the right direction.

Referring to FIG. 15, the same transmitted serial modulation waveform 18is offered as a reference. We already know the first two symbols 18 ₁and 18 ₂, and we are working off of the new error signal 140 from theprevious step. We have a good estimate of the multipath response 130₁₋₅, which is again refined with the benefit of the error signal 140based upon the previously demodulated symbol.

The next step is to estimate the third symbol 141, again by driving theerror signal 140 to zero. The resulting error signal 142 is shown next,which incorporates the effects of multipath, as estimated, on thedemodulated third symbol 18 ₃. After using the third symbol 18 ₃ and thenew error signal 142 to again update the multipath estimate 130 ₁₋₅, thefourth symbol 146 is estimated. A new error signal 148 is generated.

Again, the same transmitted serial modulation waveform 18 is offered asa reference. We already know the first four symbols 18 ₁-18 ₄ fromearlier in the process. We are working off of the new error signal 148from the previous step. Again, we have a good estimate of the multipathresponse 130 ₁₋₅, again refined using the new error signal 148 and thefourth symbol 18 ₄, just demodulated.

The next step is to estimate the fifth symbol 149, again by driving theerror signal 148 to zero. The resulting error signal 150 is shown next,which again incorporates the effects of multipath, as estimated, on thisnewest demodulated symbol. After using the fifth symbol 18 ₅ and the newerror signal 150 to again update the multipath estimate, the last symbol152 is estimated, and a new error signal 154 is generated.

The flatline of error signal 154 indicates successful demodulation, asillustrated in FIG. 16. Any deviation at this point from zero would bedue to one or more of the following causes. Noise in the receivedsignal; errors in the multipath estimate, which is normal in noisychannels but limited with respect to equalizer tap noise due to theabsence of compound equalizer echos; and demodulation errors, which areexpected when operating near the SNR threshold which is much lower thanthat experienced by equalizer-based systems in severe multipathenvironments. Any error left can be used to drive an adaptive multipathor channel estimation.

In another embodiment of the digital receiver, adaptive algorithms areapplied to both processes, i.e., channel estimation and symbolestimation. The first part of this method is an adaptive channelestimation process illustrated in FIG. 17. In this digital receiver120′, the received signal waveform r(n) is stored in the memory 126 as areceived signal vector r(n,k) whose depth is represented by index k. Anadaptive algorithm 170 may be part of the channel estimator 172. It isassumed that the transmission modulation waveform s(n) is known andstored as a vector s(n,k) also indexed in depth by sample index k. Thefollowing convention applies to each element of the transmissionmodulation-waveform vector s(n,k):s(n,k)=s(n+k)

This same convention applies to all vector variables using (n,k)arguments throughout this document. The vector modulation waveforms(n,k) is applied to an estimate ĥ(n,m) of the transmission-channelsampled-time response h(n,m). For purposes of initialization, thetransmission-channel sampled-time response-estimate ĥ(n,m) may beinitialized, at the beginning of the process, to unity-gain at m=0 andzero response at all other values of m.

When the vector modulation waveform s(n,k) is applied to thechannel-response estimate ĥ(n,m), the result is an estimate vector{circumflex over (r)}(n,k) of the corresponding received waveform vectorr(n,k). These two vectors are subtracted in the summing network 128,resulting in the error signal vector ē(n,k)= {circumflex over (r)}(n,k)−r(n,k). This error signal drives the adaptation process 170, whichmodifies the channel-response estimate ĥ(n,m) in the channel estimator172 in such a manner as to cause the error vector ē(n,k) to converge onthe corresponding zero vector.

Any number of adaptive algorithms may be used to gradually modify thechannel response vector estimate ĥ(n,m) towards a successively moreaccurate representation of the channel response vector h(n,m). The LMSalgorithm is known for its advantages in tracking non-stationaryprocesses and is used, for that reason, as an example. The LMS algorithmrequires a convergence coefficient μ. In this case, the convergencecoefficient is defined at every time-sample point n over the vectordepth index k. The vector convergence coefficient is denoted μ_(h)(n,k). An LMS adaptation recursion equation suitable for adaptationat every time sample n is

${\hat{h}( {{n + 1},m} )} = {{\hat{h}( {n,m} )} - {\sum\limits_{k = {k_{\min} + m_{\max}}}^{k_{\max} + m_{\min}}\;{{\mu_{h}( {n,{k - m}} )}{e( {n,k} )}{s( {n,{k - m}} )}}}}$

An advantageous feature of the present invention is contained in thesecond part of this method, which is the progressive adaptive estimationof the transmission modulation waveform s(n). An adaptive algorithm 180may be part of the symbol estimator 172. As best illustrated by thedigital receiver 120″ in FIG. 17, an adaptive process 180 is used toconverge on the most likely modulation waveform when the channelresponse approximation vector ĥ(n,m) in the channel estimator 172′ issufficiently known to be a sufficiently valid approximation of thechannel response vector h(n,m).

In this digital receiver 120″, the received signal waveform r(n) isagain stored in the memory 126 as a received signal vector r(n,k), whosedepth is represented by index k. It is assumed that the channel responsevector h(n,m) is sufficiently known and stored as a vector ĥ(n,m) alsoindexed in depth by sample index k. An estimate ŝ(n,k) of the vectormodulation waveform s(n,k) is applied to the stored channeltime-response vector-estimate ĥ(n,m). For purposes of initialization,the estimate ŝ(n,k) of the transmitted modulation waveform may beinitialized, at the beginning of the process, to all zeroes.

When the vector modulation-waveform approximation ŝ(n,k) is applied tothe channel-response estimate ĥ(n,m), the result is an estimate vector{circumflex over (r)}(n,k) of the corresponding received waveform vectorr(n,k). These two vectors are subtracted in the summing network 128,resulting in the an error signal vector

${\overset{\_}{e}( {n,k} )} = {{\overset{\bigwedge}{\overset{\_}{r}}( {n,k} )} - {{\overset{\_}{r}( {n,k} )}.}}$This error signal drives the adaptation process, which modifies theestimate ŝ(n,k) of the vector modulation waveform s(n,k) in such amanner as to cause the error vector ē(n,k) to converge on thecorresponding zero vector.

Again, any number of adaptive algorithms may be used to gradually modifyvector modulation waveform approximation vector ŝ(n,k) towards asuccessively more accurate reproduction of the transmitted modulationwaveform vector s(n,k). Again, the LMS algorithm is known for itsadvantages in tracking non-stationary processes and is used, for thatreason, as an example. The LMS algorithm requires a convergencecoefficient μ. In this case, the convergence coefficient is defined atevery time-sample point n over the vector depth index k. The vectorconvergence coefficient is denoted μ _(s)(n,k). An LMS adaptationrecursion equation suitable for adaptation at every time sample n is

${\hat{s}( {{n + 1},{k - 1}} )} = {{\hat{s}( {n,k} )} - {\sum\limits_{m = m_{\min}}^{m_{\max}}\;{{\mu_{s}( {n,k} )}{e( {n,{k - m}} )}{\hat{h}( {n,m} )}}}}$

The process is completed through the selection of a suitable delay indexk_(d) from which to generate a modulation waveform estimate ŝ(n+k_(d))suitable for demodulation through demodulator 184. This demodulationprocess yields an estimate {circumflex over (x)}(n+k_(d)) of theoriginal corresponding data sequence element x(n+k_(d)).

What has just been described is a method of adaptively converging on anestimate ŝ(n) of the modulation waveform s(n). However, many serialdata-modulation processes are linear. In each of these cases, anappropriate substitution of variables serves to convert this method intoan equivalent form where adaptation is applied directly to an estimate{circumflex over (x)}(n) of the modulation data-sequence x(n).

An example of such a system where this is possible is the 8-VSBmodulation applicable to the ATSC standard for terrestrial televisionbroadcast. Such direct estimation of the modulation data-sequenceresults in a significant advantage in computational efficiency. Suchdirect estimation of the modulation data-sequence through thesubstitution described is also relevant and applicable to the remainderof this disclosure.

Further savings in computational efficiency may be realized byconsidering the restrictions on modulation symbol-states associated witha modulation data-sequence x(n) specific to a given modulation system inquestion. Again, referring to the 8-VSB ATSC DTV example, the modulationdata-sequence in this case is limited to 8 states (four positive statesand four negative states, namely: −7, −5, −3, −1, 1, 3, 5 and 7).

An improvement in bit-error-rate (BER) performance is achievable asfollows. In many modulation systems, linear modulation applies andforward error correction is employed, whether by trellis codedmodulation, other convolutional coding or by block coding. In thesecases, features of decision-feedback adaptation are introduced into theprocess by which the modulation waveform estimate (or the data sequenceestimate) is caused to adaptively converge on the transmitted modulationwaveform (or the original data sequence).

Specifically, Viterbi or other MLSE processes are applied to carefullyselected elements of the modulation-waveform approximation vectorŝ(n,k). As such, a more reliable estimate of the transmitted modulationwaveform and of the original data sequence is generated.Correspondingly, adaptation time is reduced. In many cases, complexityis reduced in the process of reducing the number of required adaptationiterations.

The two components of this method described above and respectivelyillustrated in FIGS. 17 and 18 may also be combined into a signaldigital receiver 120′″. Referring now to FIG. 19, this aspect of thepresent invention includes provisions for I&Q (I and Q sampler 192, i.e,for A/D conversion of the real and imaginary components of the RFwaveform, as well as provisions for timing recovery 196. In this case,timing recovery may be based on correlation (via correlator 194) againstan embedded reference waveform. Timing recovery is used to drive the I&Qsampling process as well as the timing of convergence coefficients μ_(h)(n,k) and μ _(s)(n,k) used in the adaptive algorithms 190, which maybe included within the symbol estimator 182, or within the channelestimator 172. A modulator 183 and a demodulator 184 are also part ofthe digital receiver 120′″.

FIG. 20 illustrates another embodiment of the digital receiver 120″″ forsimultaneously demodulating a plurality of received digitally modulatedsignals subjected to multipath propagation impairments.

The process of joint adaptation of the channel time-responseapproximation ĥ(n,k) and of the transmitted modulation waveformapproximations ŝ(n,k) will now be discussed.

Various methods may be employed to realize practical joint adaptation.The first method of realizing practical joint adaptation involves theadaptation of the transmitted modulation-waveform vector approximationŝ(n,k) simultaneously with that of the vector channel time-responseapproximation ĥ(n,k).

In a “blind” sense, ĥ(n,k) may be initialized with a single unitamplitude sample surrounded by all zero amplitude samples. In a“trained” sense, the vector channel time-response approximation ĥ(n,k)may be approximated through initial training based on a trainingwaveform.

The second method of realizing practical joint adaptation comprisesalternating adaptation of large segments with respect to depth index k.For example, the vector channel time-response approximation ĥ(n,k) isfirst initialized with a received training waveform. This approximationis held constant while the transmitted modulation-waveform vectorapproximation ŝ(n,k) is adaptively estimated over an appropriately sizedsegment of samples with respect to depth index k.

The size of this segment may be chosen appropriately with respect tominimum stationary intervals applicable to anticipated multipath. Thistransmitted modulation-waveform vector approximation ŝ(n,k) isinitialized in its adaptation process with the known training waveform.At the conclusion of the adaptive process used to converge on thetransmitted modulation-waveform vector approximation ŝ(n,k), the vectorchannel time-response approximation ĥ(n,k) adaptation is resumed. Theprocess continues back-and-forth between adaptive convergence of ŝ(n,k)over some interval in domain k and subsequent vector channeltime-response approximation ĥ(n,k) adaptation.

A third method of realizing joint adaptation involves transformation ofthe modulation-waveform vector-approximation recursion-equations forŝ(n,k) into a single equation in one unknown variable. In other words,linear combination or estimation is being performed. Such an equation isformulated from the vector channel time-response approximation ĥ(n,k).This equation is applied to known samples of ŝ(n,k) to solvesuccessively for unknown samples, one at a time. The approximationĥ(n,k) is updated either every time sample n or in appropriately sizedsegments.

A fourth method involves the use of an adaptation convergencecoefficient μ_(s)(n,k) scaled in magnitude over depth index k foradaptive convergence of the modulation waveform approximation ŝ(n,k).

All of these methods are subject to the caveats described above. Theseinclude operation in the data-sequence domain “ ŝ(n,k)” as opposed tooperation in the modulation-waveform domain “ ŝ(n,k)”. These caveatsalso include the introduction of “decision” activity in theapproximation process in the interest of BER performance and in theinterest of reduced system complexity.

There is a significant advantage associated when operating in thedata-sequence domain {circumflex over (x)}(n,k) as opposed to operatingin the modulation-waveform domain ŝ(n,k). This advantage is one ofreduced complexity. This advantage is owed to the fact that, whenoperating in the data-sequence domain {circumflex over (x)}(n,k), therecursion equations used for adaptation need only be exercised at thesample points at which data-sequence samples are present.

In summary, the use of joint modulation waveform (or data sequence)adaptation approximation and channel time-response adaptationapproximation has several clear advantages over conventionalequalization techniques. The method of adaptive convergence on channeltime-response is advantageous over adaptive convergence oninverse-channel equalization response in that adaptation is limited intime to the duration of the channel time-response; a shorter convergencetime is required as a consequence; required accuracy is limited to thatof a fewer number of channel time-response taps as opposed to a greaternumber of equalizer taps otherwise necessary to accomplish substantialchannel-inverse filtering; and channel estimation is alwaysmathematically realizable as opposed to inverse-channel responseestimation, which is sometimes not mathematically realizable in apractical FIR filter.

Similarly, the use of adaptive algorithms, such as LMS, to estimatetransmitted modulation waveforms or original data sequences is superiorto MLSE methods in the following respects: there is no requirement tomaintain surviving trellis paths or to calculate associated metrics;complexity does not necessarily increase with multipath delay intervals;and complexity is reduced to manageable levels in extreme cases.

Additionally, the advantages of conventional methods are applicable tothe method of joint adaptive approximation of modulation waveforms ordata sequences and channel time-responses. These advantages include: theability to exploit training (data) sequences or (modulation) waveformsfor improved performance as is the case for “trained” equalization;ability to initialize from a “blind” start as is the case for “blind”equalization; the ability to improve performance through “decision”processes as is the case for “decision-feedback” equalization; and theability to improve performance through decisions based on convolutionalencoding as is the case for MLSE demodulation.

FIG. 20 illustrates the extension of joint channel and modulationwaveform estimation to cases where at least two modulation waveformsapplied to two distinct propagation channels are received jointly. Inthis case, the disclosed methods apply to the reception of eachmodulation waveform independently through each propagation channel. Therecursion equations described above are applicable subject toappropriate sub-scripting with respect to index of modulation origin (1through N).

The received modulation waveforms are jointly recoverable under thefollowing conditions: independent training waveforms are employed ateach modulator, s₁(n) through s_(n)(n), which have sufficientlyfavorable autocorrelation and cross-correlation properties (near-impulseautocorrelation and very low cross-correlation); and sufficient SNR isavailable.

Many modifications and other embodiments of the invention will come tothe mind of one skilled in the art having the benefit of the teachingspresented in the foregoing descriptions and the associated drawings.Therefore, it is to be understood that the invention is not to belimited to the specific embodiments disclosed, and that modificationsand embodiments are intended to be included within the scope of theappended claims.

1. A method for demodulating a received digitally modulated signalsubjected to multipath propagation impairment, the method comprising:estimating the multipath propagation impairment of the receiveddigitally modulated signal during at least one interval of clear-channelreception; estimating at least one symbol of the received digitallymodulated signal; adjusting the at least one estimated symbol based uponthe estimated multipath propagation impairment to generate an estimateof the at least one symbol as impaired by the multipath propagation;generating at least one error signal by comparing the estimate of the atleast one symbol as impaired by the multipath propagation to thereceived digitally modulated signal; and using the at least one errorsignal for estimating remaining symbols to be demodulated.
 2. A methodaccording to claim 1, further comprising using the at least one errorsignal for refining the estimated multipath propagation impairment.
 3. Amethod according to claim 2, further comprising: estimating at least onenext symbol; and adjusting the estimate of the at least one next symbolbased upon the refined estimated multipath propagation impairment forgenerating an estimate of the at least one next symbol as impaired bythe multipath propagation.
 4. A method according to claim 3, furthercomprising refining the at least one error signal by comparing theestimate of the at least one next symbol as impaired by the multipathpropagation to the received digitally modulated signal.
 5. A methodaccording to claim 4, wherein refining the at least one error signalfurther comprises comparing the estimate of the at least one next symbolas impaired by the multipath propagation to the at least one errorsignal resulting from at least one previous comparison.
 6. A methodaccording to claim 1, wherein estimating the multipath propagationimpairment and estimating the at least one symbol are based upon anadaptive algorithm.
 7. A method according to claim 1, wherein estimatingthe multipath propagation impairment and estimating the at least onesymbol are based upon a training waveform embedded in the receiveddigitally modulated signal.
 8. A method according to claim 1, whereinthe received digitally modulated signal comprises at least one of adigital broadcast television signal, a digital broadcast radio signal, adigital cellular telephone signal, and a digital wireless local areanetwork (LAN) signal.
 9. A method for demodulating a received digitallymodulated signal subjected to multipath propagation impairment, themethod comprising: estimating the multipath propagation impairment ofthe received digitally modulated signal during at least one interval ofbenign multipath propagation impairment; estimating at least one symbolof the received digitally modulated signal; adjusting the at least oneestimated symbol based upon the estimated multipath propagationimpairment to generate an estimate of the at least one symbol asimpaired by the multipath propagation; generating at least one errorsignal by comparing the estimate of the at least one symbol as impairedby the multipath propagation to the received digitally modulated signal;and using the at least one error signal for estimating remaining symbolsto be demodulated.
 10. A method according to claim 9, further comprisingusing the at least one error signal for refining the estimated multipathpropagation impairment.
 11. A method according to claim 10, furthercomprising: estimating at least one next symbol; and adjusting theestimate of the at least one next symbol based upon the refinedestimated multipath propagation impairment for generating an estimate ofthe at least one next symbol as impaired by the multipath propagation.12. A method according to claim 11, further comprising refining the atleast one error signal by comparing the estimate of the at least onenext symbol as impaired by the multipath propagation to the receiveddigitally modulated signal.
 13. A method according to claim 12, whereinrefining the at least one error signal further comprises comparing theestimate of the at least one next symbol as impaired by the multipathpropagation to the at least one error signal resulting from at least oneprevious comparison.
 14. A method according to claim 9, whereinestimating the multipath propagation impairment and estimating the atleast one symbol are based upon an adaptive algorithm.
 15. A methodaccording to claim 9, wherein estimating the multipath propagationimpairment and estimating the at least one symbol are based upon atraining waveform embedded-in the received digitally modulated signal.16. A method according to claim 9, wherein the received digitallymodulated signal comprises at least one of a digital broadcasttelevision signal, a digital broadcast radio signal, a digital cellulartelephone signal, and a digital wireless local area network (LAN)signal.
 17. A digital receiver comprising: a channel estimator forestimating multipath propagation impairment of a received digitallymodulated signal during at least one interval of clear-channelreception; a symbol estimator connected to said channel estimator forestimating at least one symbol of the received digitally modulatedsignal, said channel estimator adjusting the at least one estimatedsymbol based upon the estimated multipath propagation impairment togenerate an estimate of the at least one symbol as impaired by themultipath propagation; and a summing network connected to said channelestimator and said symbol estimator for generating at least one errorsignal by comparing the estimate of the at least one symbol as impairedby the multipath propagation to the received digitally modulated signal;said symbol estimator using the at least one error signal for estimatingremaining symbols to be demodulated.
 18. A digital receiver according toclaim 17, wherein said channel estimator uses the at least one errorsignal for refining the corresponding estimated multipath propagationimpairment.
 19. A digital receiver according to claim 18, wherein saidsymbol estimator estimates at least one next symbol, and adjusts theestimate of the at least one next symbol based upon the refinedestimated multipath propagation impairment for generating an estimate ofthe at least one next symbol as impaired by the multipath propagation.20. A digital receiver according to claim 19, wherein said summingnetwork further refines the at least one error signal by comparing theestimate of the at least one next symbol as impaired by the multipathpropagation to the received digitally modulated signal.
 21. A digitalreceiver according to claim 20, wherein said summing network refines theat least one error signal by comparing the estimates of the at least onenext symbol as impaired by the multipath propagation to the at least oneerror signal resulting from at least one previous comparison.
 22. Adigital receiver according to claim 17, wherein said channel estimatorcomprises an adaptive algorithm for estimating the multipath propagationimpairment; and wherein said symbol estimator comprises an adaptivealgorithm for estimating the at least one symbol.
 23. A digital receiveraccording to claim 17, wherein said channel estimator estimates themultipath propagation impairment based upon a training waveform embeddedin the received digitally modulated signal; and wherein said symbolestimator estimates the at least one symbol based upon a trainingwaveform embedded in the received digitally modulated signal.
 24. Adigital receiver according to claim 17, wherein the received digitallymodulated signal comprises at least one of a digital broadcasttelevision signal, a digital broadcast radio signal, a digital cellulartelephone signal, and a digital wireless local area network (LAN)signal.
 25. A digital receiver comprising: a channel estimator forestimating multipath propagation impairment of a received digitallymodulated signal during at least one interval of benign multipathpropagation impairment; a symbol estimator connected to said channelestimator for estimating at least one symbol of the received digitallymodulated signal, said channel estimator adjusting the at least oneestimated symbol based upon the estimated multipath propagationimpairment to generate an estimate of the at least one symbol asimpaired by the multipath propagation; and a summing network connectedto said channel estimator and said symbol estimator for generating atleast one error signal by comparing the estimate of the at least onesymbol as impaired by the multipath propagation to the receiveddigitally modulated signal; said symbol estimator using the at least oneerror signal for estimating remaining symbols to be demodulated.
 26. Adigital receiver according to claim 25, wherein said channel estimatoruses the at least one error signal for refining the correspondingestimated multipath propagation impairment.
 27. A digital receiveraccording to claim 26, wherein said symbol estimator estimates at leastone next symbol, and adjusts the estimate of the at least one nextsymbol based upon the refined estimated multipath propagation impairmentfor generating an estimate of the at least one next symbol as impairedby the multipath propagation.
 28. A digital receiver according to claim27, wherein said summing network further refines the at least one errorsignal by comparing the estimate of the at least one next symbol asimpaired by the multipath propagation to the received digitallymodulated signal.
 29. A digital receiver according to claim 28, whereinsaid summing network refines the at least one error signal by comparingthe estimates of the at least one next symbol as impaired by themultipath propagation to the at least one error signal resulting from atleast one previous comparison.
 30. A digital receiver according to claim25, wherein said channel estimator comprises an adaptive algorithm forestimating the multipath propagation impairment; and wherein said symbolestimator comprises an adaptive algorithm for estimating the at leastone symbol.
 31. A digital receiver according to claim 25, wherein saidchannel estimator estimates the multipath propagation impairment basedupon a training waveform embedded in the received digitally modulatedsignal; and wherein said symbol estimator estimates the at least onesymbol based upon a training waveform embedded in the received digitallymodulated signal.
 32. A digital receiver according to claim 25, whereinthe received digitally modulated signal comprises at least one of adigital broadcast television signal, a digital broadcast radio signal, adigital cellular telephone signal, and a digital wireless local areanetwork (LAN) signal.